In known semiconductor lasers and their methods of production, multilayer structures made of semi conductor materials, insulator materials and, if applicable, metals are usually formed by means of corresponding precipitation procedures and layer growth methods and are structured by way of commonly known methods, such as lithographic processes or etching techniques. All semiconductor lasers have in common that they have at least one active region in which stimulated emission takes place, thus enabling the function of a laser.
In the formation of the active region, there substantially are two differences in known semiconductor lasers in terms of the spatial expansion of the active region. In semiconductor laser diodes, for example, the active region is usually two-dimensional and exists in the entire area of the multilayer structure, and localization of the laser radiation is achieved by means of elements such as waveguides and/or a ridge, which are arranged above and/or below the active region and have a corresponding spatial structure. Also, it is known in semiconductor lasers of this kind to arrange optical feedback elements above and/or below the active region, which allow mode selection of the laser radiation, and to thus limit the laser radiation emitted by the semiconductor laser to a single laser mode in the ideal case.
Apart from semiconductor lasers of this kind, structures are also known in which a spatial limitation of the active region is necessary or at least appropriate. Said necessity of spatial limitation is usually caused by the problem of current spreading, which causes the pumping current to spread within the active region, thus allowing only a spatially less localized and consequently inefficient generation of laser radiation. To prevent this kind of current spreading within the active region, material is usually removed in semiconductor lasers of this kind in the course of structuring the multilayer structure, said material being removed in a material removal direction, which is generally perpendicular to a surface of a substrate of the semiconductor laser, and at least enough material or enough layers are removed for the active region to also be completely removed at least in areas along the material removal direction, the active region thus becoming spatially limited in one dimension perpendicularly to the layer expansion plane.
In semiconductor lasers of this kind, too, structuring attempts have been made that aimed at arranging an optical feedback element preferably in spatial proximity to the active region and to thus allow optical feedback and resulting mode selection of the laser radiation in the active region. It is known from US 2006/0056472 A1, for example, to provide a so called vertical grating. However, structures of this kind have proved to be disadvantageous in multiple respects. First of all, their production and corresponding production processes are complex. In addition to or precisely because of that, the percent yield of lasers that have the desired specification of being spectrally single-mode is only very low.
Moreover, for unipolar lasers, it is known from US 2005/0276298 A1 to arrange a plurality of laser cavities in a row so as to achieve mode selection via superimposition of the individual spectra. Different arrangements of grating structures can be used in this process.
Furthermore, from U.S. Pat. No. 5,982,804 A, a DFB semiconductor laser is known that has a grating structure formed by a semiconductor material. The top edge of the semiconductor grating structure can be arranged at level with the top edge of the active zone of the laser.
From documents US 2001/0036213 A1 and JP S62-295 480 A, DFB lasers are also known in which a grating structure layer is arranged in the area of the waveguide ridge and in the adjacent material removal areas.